Ground Motion Synthesis and Seismic Scenario in Guwahati City--A Stochastic Approach

Abstract

Guwahati is fast emerging as a multi-ethnic cosmopolitan city with a burgeoning population and rapid unplanned urbanization. It is a commercial hub, catering to eight states in northeast India. The city is located in the lower Brahmaputra Valley and underlain by thick alluvial deposits ranging from 25 to 600 m with granitic exposure at places. The regional seismotectonics of northeast India exhibit a rather large potential for high-magnitude earthquakes, as reported by Thingbaijam et al. (2008). Large earthquakes in the past caused widespread damage to life and property in this terrain. Presently, the urban agglomeration in this area is quite vulnerable to possible future great earthquakes. Given the devastation caused recently by the Sichuan earthquake of 12 May 2008 ( MW 7.9), with a death toll of nearly 70,000 and loss of property worth billions of dollars, it is imperative that we have a realistic hazard scenario for this extremely vulnerable seismic province. Seismic hazard quantification at a site is based, essentially, on understanding the site response and the propagation path of seismic waves, as well as the source characterization of a damaging earthquake. A typical deterministic seismic hazard model takes these physical attributes into account while estimating peak ground acceleration (PGA). Figure 1. ▴ Seismotectonic map of northeast India adapted from Thingbaijam et al. (2008) wherein the fault rupture zones of four scenario earthquakes have been depicted with rectangular polygons. The results of the simulations of four scenario earthquakes based on four large historical earthquakes in the region have been reported here. An extended finite-fault stochastic modeling approach has been adapted for strong ground-motion synthesis for the predicted maximum earthquakes in the terrain as reported by Thingbaijam and Nath (2008). An attempt also has been made to establish regional as well as site-specific attenuation relations for Guwahati city through successive ground-motion …